The present invention relates to magnetic recording media and in particular, to a magnetic recording medium having a metal thin film formed on a base layer as a perpendicular magnetic recording layer.
Recently, considerable effort has been given to the development of perpendicular magnetic recording media. Such recording media offer an advantage over conventional magnetic tapes or disks in that they can store data information at a high recording density to provide very large storage capabilities.
A disk-shaped magnetic recording disk of the type known as a floppy disk or diskette can include a ferromagnetic metal thin film which is formed, by a sputtering or vapor deposition method, on a flexible base layer to serve as a perpendicular recording layer. The recording layer may be a thin film of cobalt-chromium based alloy, or a thin film of oxide having a magnetoplumbite crystal structure, such as ferrite barium, ferrite strontium, etc. These kinds of ferromagnetic substances are too hard to obtain the elasticity enough to keep a smoothness in mechanical touch between the magnetic head and the recording layer. Accordingly, not only the recording layer but also the magnetic head, itself, will be damaged at its tracing portion which is in continuous and direct contact with the recording layer of rotating disk in the data read/write mode.
On each recording layer is thus provided a protective layer which is usually comprised of a nonmagnetic insulative layer of a suitable mechanical strength. Silicon oxide, aluminum oxide, or titanium nitride has been considered as a common substance for the protective layer. Use of silicon oxide has been proposed in Japanese Patent Disclosure (KOKAI) No. 60-61920, for example. Use of aluminum oxide protective layer has been disclosed in Japanese Patent Disclosure (KOKAI) No. 58-91530. Any of these substances, however, cannot serve, by itself, as a complete protective layer for the perpendicular magnetic recording disk for the following reasons.
It is strictly required for the protective layer of the perpendicular magnetic disk that the thickness thereof is set to a specific value, which is suitable to make data read/write operation of the magnetic disk most effective. The thickness of the protective layer defines a requisite distance between the recording layer and the track-tracing portion of the magnetic head. Since the recording density of the perpendicular magnetic disk is very high compared with the conventional disk, when a data signal recorded is reproduced whose frequency is high and thus which could not be stored in the conventional magnetic disk, the requisite distance between the recording layer and the magnetic head must be precisely controlled not to be larger than that specific distance.
In this case it is very difficult to properly increase the hardness of the protective layer without also increasing its thickness. If the protective layers are too hard as compared to the magnetic head, the magnetic head will be worn due to continuous contact with the protective layer. Therefore, a protective layer made of aluminum oxide is not considered suitable for a perpendicular magnetic recording medium. For this reason, according to a perpendicular magnetic recording medium disclosed in Japanese Patent Disclosure (KOKAI) No. 58-91530, a liquid lubricant must be impregnated in an aluminum oxide protective layer to improve the head traceability.
If, on the other hand, the protective layer is too soft, it will be easily abraded due to continuous tracing of the magnetic head. It follows that the recording layer can no longer be protected from wear in the data recording or reproduction mode. A protective layer made of silicon oxide or titanium nitride can for no longer period remain as a good protective layer for a perpendicular magnetic recording medium.